Vn. Glazkov et al., ELECTRON-SPIN-RESONANCE IN THE DOPED SPIN-PEIERLS COMPOUND CU1-XNIXGEO3, Journal of physics. Condensed matter, 10(35), 1998, pp. 7879-7896
Electron spin-resonance studies of the Ni-doped spin-Peierls compound
CuGeO3 have been performed for the frequency range 9-75 GHz and temper
ature interval 1.3-20 K. An anomalous temperature dependence of the g-
factor below the spin-Peierls temperature was observed for doped sampl
es. At low temperatures the g-factor is much smaller than the value ex
pected for Cu2+ and Ni2+ ions and is much more anisotropic than for an
undoped crystal. This anomaly is explained by the formation of magnet
ic clusters around the Ni2+ ions within a nonmagnetic spin-Peierls mat
rix. The formation of magnetic clusters is confirmed by the observatio
n of a nonlinear static magnetic susceptibility at low temperatures. T
he reduction of the spin-Peierls transition temperature was found to b
e linear in the dopant concentration x over the range 0 less than or e
qual to x less than or equal to 3.2%. The transition into the antiferr
omagnetically ordered state, detected earlier by neutron scattering fo
r x greater than or equal to 1.7%, was studied by means of ESR. For x
= 3.2% a gap in the magnetic resonance spectrum is found below the Nee
l temperature and the spectrum is well described by the theory of anti
ferromagnetic resonance based on the molecular-field approximation. Fo
r x = 1.7% the spectrum below the Neel point remained gapless. The gap
less spectrum of the antiferromagnetic state in weakly doped samples i
s attributed to the small value of the Neel order parameter and to the
magnetically disordered spin-Peierls background.